In the recent years, applications of wireless waves and microwaves revealed infinite potentiality, as for example mobile communications and wireless networks et cetera are the most popular industry. The trend of this one field is to develop towards higher frequencies, in future millimeter and even sub-millimeter wavelengths will be used. Usually, because of the lack of wave source and detector, therefore applications of sub-millimeter wavelength magneto-rheological fluid are comparatively less. In the recent ten years, due to motivation and coherence by lasers, development of terahertz technology are maturing with each passing day, in Time-Domain Spectroscopy, applications of THz imaging and medical areas all displayed extremely huge development potentiality of terahertz wavelength. Besides this, THz (terahertz) communications and phase radar technologies are more important areas of research. Nevertheless, the above described applications usually requires THz optic components such as polarizer, filter, phase shifter, and wavelength selector device et cetera, so as to serve use in signal processing.
At present, in the already published documents, it is known that terahertz filter or wavelength selector device was first published in Applied Physics by C. Winnewisser et al. on March 1998, it mainly utilizes periodic holes created on metallic slabs, so when terahertz waves passes through this structure, it is just like passing through two-dimensional grating or photon crystals, wherein some wavelengths have specifically high transparency, while terahertz waves of some frequencies cannot pass through, utilizing this, function of wave selecting is achieved. Utilizing this structure as basis, T. D. Drysdale et al. utilized relative motion of two layers of one-dimensional grating to produce changes in periodic structures, and then succeeded in adjusting frequencies allowed to pass through, and H. Nemec et al. clipped dielectric materials in between the two layers of periodic structures, wherein the dielectric constant of the dielectric materials changes with change in temperature, utilizing this, through temperature adjustments, frequencies allowed to pass through can be changed. Besides this, Stephan Biber et al. also utilized the known grating technology and successfully accomplished applications of wavelength selector device under terahertz wavelengths. In spite of these wavelength selector device using photon crystals as basis (such as, filters) possesses good filtrate functions, yet its exceedingly low frequency adjusting range (<20%) or range of temperatures required for change being exceedingly high (100 K˜300 K) both increased the difficulty in its applications.
In microwaves or sub-micrometer wavelengths, utilizing liquid crystals in tunable wavelength selector device has already being discussed widely. M. Tanaka and F. Yang et al. utilized metal laminates to create Stack-Layered Liquid Crystal Cell), in reality it also utilizes electric fields to change the refraction constant of the liquid crystal and then adjusts it to one dimensional periodic structure, only it alone is suitable for use in millimeter wave ranges, due to wavelength of electromagnetic waves in terahertz wavelength being very small, hence accomplishing this structure in terahertz wavelength exists certain difficulties. Applications proclaimed by B. Lyot in 1944, utilizing combinations of phase retarding discs and linear polarizers as wavelength selector is already widely known, but to date it is not yet successfully applied in terahertz wavelengths, it is primarily because: (1) Under this wavelength, suitable phase retarding material has not yet been developed, and (2) On applications of tunable wavelength selector (or filter), it is still required to take into consideration a large enough tunable phase retardation range. In other words, even though use of phase retarder and linear polarizer combination as wavelength selector is already a known technology, yet implementing it in terahertz wavelength poses certain difficulty and limitations, if this method is used to accomplish function of tunable wavelength selecting, its phase retarder should possess a large enough continuous tunable range (>360 deg)